Heretofore, subfields were merged with the host field by the "key-croma" technique. A frame or window in the background of the host field was maintained an intense blue. The blue channel of the vidicon was monitored for a high voltage produced when the camera beam scanned the blue background area. The high blue voltage activated a video switch which substituted the subfield data for the host field. One difficulty with this prior art technique is that the foreground of the host field was limited to only minor blue coloring. The foreground blue had to be maintained below the level required to activate the video switch.
Further, only one or two subfields may be keyed on the contents of the host field. The shades-of-blue available for differentiating between subfield windows is limited.